The large-scale industrial production of flexible polyurethane foams using isocyanates, polyether polyols, optionally cross linkers and other suitable additives has long been known and is described, for example, in Becker/Braun, Kunstoff-Handbuch (Plastics Handbook), Volume 7, Polyurethanes, published by Carl Hanser, Munich, Vienna, 2nd edition 1983.
Depending on the reactivity of the raw materials, a differentiation is made between hot cured flexible foams (hereinafter referred to as conventional flexible foams) and cold cured flexible foams (hereinafter referred to as high resilience (HR) foams), the concepts being derived from foaming in molds. When conventional flexible foams are produced by the mold method, the foam, because of the low reactivity of the raw materials, must be heated in the mold at an elevated temperature in order to complete the cross linking; these foams therefore are referred to as hot cured foams.
The development of highly reactive polyether polyols and optionally the additional use of cross-linking agents, on the other hand, enable foams to be produced in the mold which, because of the rapid curing, require little input of heat. Such foams therefore are named cold cured foams.
Aside from producing the foam in a mold, it is also possible to carry out the foaming by the slabstock method, for which the concepts of conventional flexible foam and high resilience foam have also become established.
Due to different starting raw materials, high resilience foams have typical physical properties different from those of conventional flexible foams.
The high resilience foams have:
(a) a latex-like feel, PA1 (b) a higher elasticity than conventional flexible foams; therefore, these foams are referred to as "highly resilient foams", PA1 (c) compression hardness characteristics differing from those of conventional flexible foam (a higher SAG factor) and thus offering a better sitting comfort when used as an upholstery material (furniture foam), PA1 (d) better continuous use properties with only a slight fatigue tendency, which is of great interest particularly in the automobile sector, PA1 (e) because of its melting behavior, a lower flammability than the conventional flexible foams, PA1 (f) a more advantageous energy balance and shorter molding times during mold (foaming) production in the molding operation. PA1 A. polymer polyols: these are highly reactive polyols containing a dispersion of a copolymer based on styrene and acrylonitrile; PA1 B. PHD polyols: these are highly reactive polyols containing polyurea, also in dispersed form; and PA1 C. PIPA polyols: these are highly reactive polyols containing a polyurethane (formed by an in situ reaction between an isocyanate and an alkanolamine in a conventional polyol) in a dispersed form. PA1 n has an average numerical value of 2 to 11, PA1 m has an average numerical value of 1 to 6, with the proviso that the ratio of p=(n+m+2)/m is 1 to 12, are added as stabilizers.
In particular, a flexible polyurethane foam is produced by reacting a mixture of polyol, polyfunctional isocyanate, amine activator, tin catalyst, stabilizer, blowing agent (either water for forming carbon dioxide and/or the addition of physical blowing agents), optionally with the addition of flame retardants, cross-linking agents or other customary processing aids.
In contrast to conventional flexible foams, high resilience foams are produced from highly reactive polyols and, in addition, low molecular weight cross-linking agents. Higher functional isocyanates (so-called "crude MDI") may act as a cross-linking agent. Accordingly, there is already a reaction between isocyanate groups and hydroxyl groups during the expansion phase (formation of carbon dioxide from --NCO and H.sub.2 O) of the foam. This rapid polyurethane reaction initially leads to an increase in viscosity and then to a relatively high inherent stability of the foam during the blowing process.
For high resilience foam therefore, stabilizers are required, which control the cell size and the cell size distribution, and stabilize the subsurface area as well. Furthermore, compared to conventional flexible foams, high resilience foams have a higher proportion of closed cells, which must be opened mechanically (crushed) after removal of the foam from the mold. In addition, the high resilience foam has an irregular cell structure and, as a rule, coarser cells than conventional flexible foam, which contributes significantly to its properties named above.
As polyols, highly reactive polyols are used. These are trifunctional polyols, which have a high weight molecular weight of usually between about 4,800 and 6,500 g/mole and contains at least 70% (up to 95%) primary hydroxyl groups, so that their OH number is between 36 and 26. To the extent of up to 90%, these polyols are built up from propylene oxide. However, they contain primary hydroxyl end groups, which have resulted almost exclusively from the addition reaction of ethylene oxide. The primary hydroxyl groups are far more reactive towards isocyanate groups than are the secondary hydroxyl groups of the polyols used for conventional flexible foams. Their OH numbers usually are between 56 and 42 for weight molecular weights of 3,000 to 4,500.
The highly reactive polyols are obtained by the polyaddition reaction between propylene oxide or ethylene oxide and compounds of higher functionality, such as glycerin or trimethylolpropane, in the presence of basic compounds.
The so-called filled polyols represent a further class of highly reactive polyols. Aside from the characteristic data listed above, the latter are distinguished by the fact that they contain up to 40% or more of solid organic fillers in a dispersed distribution. It is customary to differentiate between:
The proportion of solids, which preferably lies between 5 and 40% depending on the application, is responsible for improved cell opening, so that the polyol can be reacted in a controlled manner particularly with the TDI to avoid shrinkage of the foams. The solid acts as an essential processing aid. A further function is to control the hardness by the solids content, since higher solid contents increase the hardness of the foam.
The formulations with solids-containing polyols have a clearly lower inherent stability and therefore require, aside from the chemical stabilization by the cross-linking reaction, a physical stabilization.
Depending on their solids content, the polyols are used alone or in admixture with the unfilled polyols named above.
As isocyanates, TDI (a 2,4- and 2,6-toluylene diisocyanate mixture of isomers), as well as MDI (4,4'-diphenylmethane diisocyanate) are used. Aside from the 4,4'-isomer, the so-called "crude MDI" or "polymeric MDI" also contains 2,4'- and 2,2' isomers, as well as polynuclear products. Binuclear products, consisting predominantly of mixtures of 2,4'-and 4,4'- isomers of their propolymers, are referred to as "pure MDI".
Different isocyanates are frequently used for slabstock foams and molded foams. For example, pure TDI (various mixtures of 2,4- and 2,6 isomers), in combination with solids-containing polyols, are usually used as polyfunctional isocyanate in high resilience slabstock foam systems. Moreover, modified TDI types are used in combination with highly reactive unfilled polyols. The German patents 25 07 161 and 26 03 498 also disclose the use of crystalline polyhydroxy compounds as cross-linking agents for slabstock foaming, in combination with highly reactive unfilled polyols, as well as TDI, trimerized TDI or also TDI/MDI mixtures. On the other hand, pure MDI formulations are normally not used for producing high resilience slabstock foams.
The formulations based on pure TDI, initially developed for foaming in molds, had the disadvantage of a narrow processing latitude. However, formulations based on TDI 80 admixtures with crude MDI (up to 20% or more), in combination with filled polyols, permit reliable foaming. Systems based on TDI with crude MDI, usually in the ratio of 70:30 to 40:60 and in combination with unfilled polyols, are also generally in use at the present time.
In addition, formulations based solely on MDI without admixture of TDI and using unfilled polyols are also found. For these, the ratio of the 2,4'- to 4,4'- isomers as well as the ratio of monomer to polymer can be varied within a wide range.
As amine activators, preferably tertiary amines are used, such as triethylenediamine (TEDA), or the bis-2-dimethylaminoethyl ether (BDE). Many formulations are based on a combined catalysis of these two compounds. However, other common amines are also possible; the amount used usually is between 0.05 and 0.2 parts per 100 parts of polyol.
Multifunctional compounds, which react with isocyanates, are referred to as cross-linking agents. Hydroxyl terminated or amine terminated substances, such as glycerin, triethanolamine (TEOA), diethanolamine (DEOA) and trimethylolpropane are suitable. They are used in concentrations of 0.5 to 2.0 parts per 100 parts of polyol, depending on the formulation; however, they may also be used in other concentrations. When crude MDI is used for foaming in molds, it also assumes a cross-linking function. As the amount of crude MDI is increased, the content of low molecular weight cross linking agent can be decreased correspondingly.
The polyurethane reaction generally is catalyzed by the addition of tin activators. Either dibutyl tin dilaurate (DBTL) or also tin(II) octoate is used, usually in amounts of between 0.01 and 0.3 parts per 100 parts of polyol; these concentrations, may, however, also be different.
Blowing agents are divided into chemical and physical blowing agents. Chemical blowing agents include water, the reaction of which with the isocyanate groups leads to the formation of carbon dioxide. The apparent density of the foam is controlled by the amount of water added, amounts between 1.5 and 4.0 parts per 100 parts of polyol being preferably used. Moreover, additional physical blowing agents, such as fluorochlorohydrocarbons, methylene chloride, acetone, 1,1,1-trichloroethane, etc., can also be used in addition.
Furthermore, for the production of high resilience foam, stabilizers are required for controlling cell size and cell size distribution and for regulating the subsurface area. In comparison to conventional flexible foam stabilizers, they generally have only weakly stabilizing properties. The requirements to be met by stabilizers differ, depending on whether they are to be used for slabstock or molded foam production.
For the slabstock process, aside from the stabilization of the foam, the necessary cell opening at the correct time is the actual problem. If the polymerization reaction has proceeded so far at the end of the expansion that the block is already fully stabilized chemically, opening can usually no longer be carried out. The whole block will therefore shrink. If the cell opening takes place prematurely, then this leads either to a collapse of the foam or, if the system is relatively stable inherently, to a foam, which can a shrink a few hours after it is produced. With the help of a suitable stabilizer, the time, as well as the intensity of the cell opening can be controlled. Moreover, the stabilizer should control the cell structure and, in particular, the subsurface area (particularly important for molded foam). The high resilience foam should have a slightly coarser cell and an irregular cell structure so that the special physical properties can be attained.
The requirements to be met by a stabilizer for high resilience slabstock foam therefore primarily are controlled foam stabilization, cell opening at the appropriate time, cell regulation and control of the cell size distribution.
There are additional requirements for producing high resilience molded foam. The expanding reaction mixture must negotiate relatively wide flow paths, in order to fill the whole volume of the mold. Frictional resistance at the mold walls easily leads to destruction of whole cell structures, so that cavities are formed under the foam skin. This defect also occurs when foam must flow around parts, which have been inserted for reinforcement. A further critical zone is the region of the vent holes. Excess blowing gas, flowing at too high a rate past the cell structures, causes partially collapsed zones.
Moreover, the quality of the foam skin is evaluated critically.
To summarize, a stabilizer for high resilience molded foam must meet the following requirements: sufficient stabilization of the foam, stabilization against the effects of shear forces, stabilization of the subsurface area and of the skin, control of the cell size and the cell size distribution, avoidance of an increased proportion of closed cells.
The general requirements, which are to be met, in addition, by a stabilizer, are a high effectiveness, that is, the stabilizer should develop its optimum effectiveness readily at low concentrations. Furthermore, the processing latitude should be large, that is, the concentration range, in which the stabilizer can be used, should, as far as possible, be large, so that slight changes in the formulation of the foam can be carried out without problems.
Because of the large number of possible raw materials, there are very many different formulations for producing high resilience foams. As a result, the above-mentioned requirements, which a stabilizer must fulfill, are strongly dependent on the system employed. Accordingly, special stabilizers must be developed for the respective system.
In principle, typical high resilience foam stabilizers are polymers based on polysiloxanes, which are modified more or less by suitable organic groups. The chain length of polysiloxanes, suitable for high resilience foam, generally are shorter than the chain length of stabilizers used for conventional flexible foams.
Several possibilities and components are thus available for adapting the structure of the stabilizer to the respective requirements.
According to the present state of the art, essentially two groups of high resilience foam stabilizers are used:
Unmodified siloxanes are employed particularly in inherently stable systems. In this connection, particularly the German patent 25 33 074 and the German Offenlegungsschrift 22 21 should be mentioned.
On the other hand, in formulations, in which real physical stabilization is required in addition (such as formulations based on filled polyols), organo-modified siloxanes are the more important. Suitable polysiloxane-polyoxyalkylene copolymers are disclosed, for example, in the U.S. Pat. Nos. 3,741,917 and 4,031,044. Other suitable organo-modified siloxanes are given in the following Table:
______________________________________ Modifying Group Reference ______________________________________ cyanoalkyl U.S. Pat. No. 3 952 038 cyanoalkoxyalkyl German Auslegeschrift 24 02 690 sulfolanyloxyalkyl U.S. Pat. No. 4 110 272 morpholinoalkoxyalkyl U.S. Pat. No. 4 067 828 t-hydroxyalkyl U.S. Pat. No. 4 039 490 chloropropyl German Patent 36 26 297 chloromethyl German Offenlegungsschrift 27 36 138 linear alkyl European Patent 0 037 067 branched alkyl European Patent 0 243 131 aralkyl German Patent 23 56 443 ______________________________________
In spite of this large number of stabilizers, it is not possible to fulfill all the requirements of the application. Up to now, no stabilizers are known, which can be used universally in all high resilience foam systems. Moreover, the problem of finding a balance between the formation of a stable and, at the same time, a very open-celled foam, represents a special challenge. In addition, new, previously unknown problems arise constantly, the solution of which cannot be accomplished or can be accomplished only incompletely with the known stabilizers of the state of the art.
The present invention therefore is concerned with the task of finding new stabilizers, which are suitable for the production of high resilience polyurethane foams and are in a position to better solve the already known problems or to solve new problems for the first time.
One problem of the high resilience foams used in the automobile sector consists therein that they contribute to undesirable fogging. The formation of a light-scattering deposit on the inner glass surfaces of an automobile is referred to as fogging. The primary source of this deposit are volatile components of the polymeric materials, which are used for outfitting the interior of the vehicle and, because of the temperature condition, evolve gases in the vehicle, which then condense on the relatively cooler glass surfaces. The automobile industry therefore is interested in reducing such fogging deposits as far as possible.
The volatile components in the high resilience foam are the cause of the fogging deposits, insofar as the latter are caused by the high resilience foam. A not inconsiderable portion of the volatile components originates from the stabilizer. These volatile components are contained in every stabilizer mixture as a result of the manufacturing conditions.
It is therefore an object of the present invention to develop new stabilizers, in which these volatile components' portions are reduced or eliminated completely.
Moreover, the present invention is concerned quite generally with the problem of developing stabilizers with improved properties. These properties include, in particular:
An improved open-celled nature of the foam and, with that, better crushing behavior with, at the same time, adequate stabilization, higher effectiveness, wider processing latitude, better skin quality of the molded foams, products, which can be used universally, that is, for high resilience slabstock foam systems as well as for high resilience molded foam systems.